A Ratiometric Fluorescent Probe Based on RhB Functionalized Tb-MOFs for the Continuous Visual Detection of Fe3+ and AA

In this study, a red-green dual-emitting fluorescent composite (RhB@MOFs) was constructed by introducing the red-emitting organic fluorescent dye rhodamine B (RhB) into metal-organic frameworks (Tb-MOFs). The sample can be used as a ratiometric fluorescent probe, which not only avoids errors caused by instrument and environmental instability but also has multiple applications in detection. The results indicated that the RhB@MOFs exhibited a turned-off response toward Fe3+ and a turned-on response for the continuous detection of ascorbic acid (AA). This ratiometric fluorescent probe possessed high sensitivity and excellent selectivity in the continuous determination of Fe3+ and AA. It is worth mentioning that remarkable fluorescence change could be clearly observed by the naked eye under a UV lamp, which is more convenient in applications. In addition, the mechanisms of Fe3+- and AA-induced fluorescence quench and recovery are discussed in detail. This ratiometric probe displayed outstanding recognition of heavy metal ions and biomolecules, providing potential applications for water quality monitoring and biomolecule determination.


Introduction
Ferric ion (Fe 3+ ), as an inorganic transition metal ion, is widely distributed in the environment and biological materials [1][2][3]. As an essential trace element, Fe 3+ plays an important part in biological processes, both clinically and environmentally, and Fe 3+ can disrupt cellular homeostasis and lead to the development of various diseases. [4,5]. A deficiency in Fe 3+ can lead to malnutrition, iron deficiency anemia (IDA) and stunted growth, while excess Fe 3+ can lead to arthritis, liver damage, kidney failure, diabetes, neurodegenerative diseases (Parkinson's, Alzheimer's) and even cancer (lung cancer, liver cancer). Therefore, the determination of Fe 3+ is key to the early diagnosis of these diseases. In addition, with the development of the economic society and chemical industry, Fe 3+ , as a common pollutant, can be released into our ecosystem through various industries and human activities, causing inevitable damage to human health and the ecological environment [6]. Therefore, sensitive detection of Fe 3+ ions plays a vital role in various biological systems and industries.
Ascorbic acid (known as vitamin C, or AA for short) is one of a class of biomolecules that are widely found in fruits and vegetables as an organic small molecule, as well as an important antioxidant that plays a considerable role in the course of daily human 2 of 14 life [7][8][9]. AA has a significant modulatory effect on neurotransmitters and enzymes of the central nervous system, with a reference range of 0.6 to 2.0 mg/dL in a seemingly healthy population [10][11][12]. Variable concentrations of ascorbic acid in biological fluids were found in clinical medical studies, where ascorbic acid is an indicator for assessing the exact amount of oxidative stress in human metabolism and plays a key role in maintaining physiological processes in humans and animals [13]. AA can be used to treat colds and physiological disorders. However, abnormalities in AA in humans can lead to associated disorders such as diarrhea, hyperacidity and kidney stones. Therefore, the analysis of AA is crucial for food safety and disease diagnosis [14].
A large number of sensors based on various designs and operating principles are being synthesized and improved to facilitate detection [15][16][17]. At present, some relevant detection methods have been developed for the detection of heavy metal ions and bio-organic substances, including electrochemistry [18,19], colorimetry [20,21], chemiluminescence [22], enzymology and capillary electrophoresis [8,23]. However, these methods have some drawbacks: low anti-interference, poor reagent stability, poor detection accuracy, many interfering factors and poor injection accuracy. Therefore, it is of great significance and urgency to explore a new chemical-sensing method to detect and sense metal ions and other detection substances with the naked eye. The fluorescence analysis method [24][25][26][27] is called "nondestructive testing" due to its nondestructive, highly sensitive, highly selective and rapid detection characteristics, which has attracted considerable attention in the field of testing and is a relatively popular testing method [28].
To address these issues, we designed a novel composite (RhB@Tb-MOFs), which was constructed by embedding fluorescent dye rhodamine B (RhB) in a porous microcrystalline Tb-based metal-organic frameworks (Tb-MOFs) and used as a novel fluorescence sensing platform (Scheme 1) for multi-domain detection (ion detection and biomolecule detection). When the sample RhB@Tb-MOFs were dispersed in an aqueous solution, the characteristic emission at 570 nm came from the RhB, resulting in improved photostability and extended fluorescence lifetime compared with free RhB. In addition, the RhB loaded inside and on the surface of the porous crystal could be used as the reference center so that the fluorescent probe has the ability for self-calibration and could well avoid environmental interference.

Characterization of the RhB@Tb-MOFs Fluorescent Probe
The XRD of the reported Tb-MOFs [29], synthetic Tb-MOFs, RhB and RhB@Tb-MOFs are shown in Figure 1a. The XRD of the synthetic Tb-MOFs was in good agreement with the reported Tb-MOFs. The XRD of the RhB@Tb-MOFs did not change significantly compared with the Tb-MOFs, indicating that the RhB modification did not damage the struc-Scheme 1. Schematic diagram of the RhB@Tb-MOFs for the continuous detection of Fe 3+ and AA.

Characterization of the RhB@Tb-MOFs Fluorescent Probe
The XRD of the reported Tb-MOFs [29], synthetic Tb-MOFs, RhB and RhB@Tb-MOFs are shown in Figure 1a. The XRD of the synthetic Tb-MOFs was in good agreement with the reported Tb-MOFs. The XRD of the RhB@Tb-MOFs did not change significantly compared with the Tb-MOFs, indicating that the RhB modification did not damage the structure of the Tb-MOFs. The absence of the characteristic peaks for RhB in the XRD of the RhB@Tb-MOFs may have been caused by the low loading of the RhB in the MOFs matrix. In addition, elemental analysis of the RhB@Tb-MOFs is shown in Figure 1b. The RhB@Tb-MOFs contained five elements, namely, C, O, N, Tb and Cl, of which N and Cl are characteristic elements of RhB. Therefore, the chemical composition of the RhB@Tb-MOFs was consistent with that of the target sample, indicating that the composite was successfully prepared. The emission spectra of different concentrations of the fluorescent dye rhodamine B (RhB) are shown in Figure S1a. As the concentration of RhB increased, the fluorescence intensity increased and then weakened due to its characteristic aggregation-induced quenching (ACQ) [30,31]. Therefore, the MOFs with the advantage of a porous structure were introduced to prevent aggregation quenching, thereby synthesizing dye@MOFs. [31,32]. The emission spectra of the Tb-MOFs and the UV absorption spectra of RhB are shown in Figure S1b, where the spectra overlapped in the range of 500~600 nm, suggesting that there may have been an energy resonance transfer between the Tb-MOF and RhB that facilitated the interaction. RhB is red-emitting material and Tb-MOFs is a green-emitting material [33][34][35], and thus, may be prepared as red-green dual-emitting composites. Therefore, the functionalized modification of the Tb-MOFs with RhB in this study has corresponding theoretical support.
In order to accurately determine the content of RhB in the RhB@Tb-MOFs, the luminescence intensity of different concentrations of RhB was measured. As shown in Figure  S2a, the content of RhB affected its luminescence and appeared red-shifted. The emission peak of RhB in the RhB@Tb-MOFs was located at 570 nm, which is within a reasonable range, indicating that the emission of RhB tended to be in the molecular state and the Tb-MOFs could effectively segregate RhB [1,36] to promote fluorescence emission. Figure S2b shows the linear fit between the concentration and intensity of RhB, yielding the relationship equation: I = 6.3033 × 10 8 C + 240.4, where C indicates the concentration of RhB and I indicates the luminescence intensity of the RhB@Tb-MOFs. Thus, the specific content of RhB in the RhB@Tb-MOFs could be calculated using the relational equation.
The emission spectra of different concentrations of the RhB@Tb-MOFs are shown in Figure S2c. The luminescence intensity of the RhB@Tb-MOFs could be adjusted according to the variation in RhB concentration. Given that the RhB@Tb-MOFs acted as a ratiometric fluorescent probe, a RhB concentration of 1 × 10 −3 M and a I545/I570 ratio of 1.5 were chosen as the target sample, and the actual loading of RhB was calculated to be 0.44% (low loading). The excitation wavelengths of the RhB@Tb-MOFs are explored as shown in Figure  S2d. Different excitation wavelengths corresponded to different ratios of RhB@Tb-MOFs in the 225~245 nm range, and the excitation wavelength of 237 nm was chosen because the ratio of RhB@Tb-MOFs was guaranteed and verified it.
The IR spectra of RhB and RhB@Tb-MOFs are shown in Figure S3a. There was no The emission spectra of different concentrations of the fluorescent dye rhodamine B (RhB) are shown in Figure S1a. As the concentration of RhB increased, the fluorescence intensity increased and then weakened due to its characteristic aggregation-induced quenching (ACQ) [30,31]. Therefore, the MOFs with the advantage of a porous structure were introduced to prevent aggregation quenching, thereby synthesizing dye@MOFs [31,32]. The emission spectra of the Tb-MOFs and the UV absorption spectra of RhB are shown in Figure S1b, where the spectra overlapped in the range of 500~600 nm, suggesting that there may have been an energy resonance transfer between the Tb-MOF and RhB that facilitated the interaction. RhB is red-emitting material and Tb-MOFs is a green-emitting material [33][34][35], and thus, may be prepared as red-green dual-emitting composites. Therefore, the functionalized modification of the Tb-MOFs with RhB in this study has corresponding theoretical support.
In order to accurately determine the content of RhB in the RhB@Tb-MOFs, the luminescence intensity of different concentrations of RhB was measured. As shown in Figure S2a, the content of RhB affected its luminescence and appeared red-shifted. The emission peak of RhB in the RhB@Tb-MOFs was located at 570 nm, which is within a reasonable range, indicating that the emission of RhB tended to be in the molecular state and the Tb-MOFs could effectively segregate RhB [1,36] to promote fluorescence emission. Figure S2b shows the linear fit between the concentration and intensity of RhB, yielding the relationship equation: I = 6.3033 × 10 8 C + 240.4, where C indicates the concentration of RhB and I indicates the luminescence intensity of the RhB@Tb-MOFs. Thus, the specific content of RhB in the RhB@Tb-MOFs could be calculated using the relational equation.
The emission spectra of different concentrations of the RhB@Tb-MOFs are shown in Figure S2c. The luminescence intensity of the RhB@Tb-MOFs could be adjusted according to the variation in RhB concentration. Given that the RhB@Tb-MOFs acted as a ratiometric fluorescent probe, a RhB concentration of 1 × 10 −3 M and a I 545 /I 570 ratio of 1.5 were chosen as the target sample, and the actual loading of RhB was calculated to be 0.44% (low loading). The excitation wavelengths of the RhB@Tb-MOFs are explored as shown in Figure S2d. Different excitation wavelengths corresponded to different ratios of RhB@Tb-MOFs in the 225~245 nm range, and the excitation wavelength of 237 nm was chosen because the ratio of RhB@Tb-MOFs was guaranteed and verified it.
The IR spectra of RhB and RhB@Tb-MOFs are shown in Figure S3a. There was no significant difference in the IR of RhB@Tb-MOFs compared with the Tb-MOFs; this was probably because the RhB loading introduced into the pores and surfaces of the Tb-MOFs was too low to have an effect on the structure of the Tb-MOFs [37] (confirmed using XRD). The N 2 adsorption test results for the Tb-MOFs and RhB@Tb-MOFs are shown in Figure S3b. The specific surface area and pore volume were 18.4238 m 2 /g and 0.0406 cm 3 /g, respectively, for the Tb-MOFs and 4.6310 m 2 /g and 0.0249 cm 3 /g, respectively, for the RhB@Tb-MOFs. By comparison, the specific surface area and pore volume of the RhB@Tb-MOFs were reduced by 74.9% and 38.7%, respectively, demonstrating that RhB was introduced into the pores or surfaces of the Tb-MOFs [38][39][40]. The thermogravimetric results for the Tb-MOFs and RhB@Tb-MOFs are shown in Figure S3c. The Tb-MOFs had three stages of weight loss (loss of water molecules, ligands, pyrolysis of the system); the RhB@Tb-MOFs had two stages of weight loss, namely, 5.7% at 200 • C and 16.1% in the 200~400 • C range, which were less than the weight loss of the Tb-MOFs in the same temperature range. The DTG curves of the Tb-MOFs and RhB@Tb-MOFs are shown in Figure S3d. The pyrolysis temperatures of Tb-MOFs were 249.6 • C, 352.8 • C and 462.8 • C, while for RhB@Tb-MOFs, they were 249.6 • C and 471.2 • C. The amount of pyrolysis of Tb-MOFs was the most obvious at 352.8 • C, while for the RhB@Tb-MOFs, it was smaller than that of Tb-MOFs at the same temperature. In addition, the pyrolysis process corresponded to the weight loss process, indicating that the RhB@Tb-MOFs had excellent pyrolysis resistance.
The SEM of the RhB@Tb-MOFs is shown in Figure S4a, in which a large number of spheres with diameters of about 3~5 µm were distributed, and the surface was relatively rough. The elemental mapping of the RhB@Tb-MOFs is shown in Figure S4b-f, and the five elements were uniformly distributed, which was consistent with the EDX test results, indicating the successful synthesis of the RhB@Tb-MOFs. The Tb-MOFs are shown as uniformly distributed circular spheres at high magnification in Figure S5a; the inset is a sphere with a smooth surface. The RhB@Tb-MOFs are also shown as uniform spheres in Figure S5b; the inset is a sphere with a rough surface (the surface attachment was RhB). The morphology of the RhB@Tb-MOFs did not change, indicating that the functionalized modification of Tb-MOFs by RhB did not affect the structure.
The UV absorption spectra of the RhB, Tb-MOFs and RhB@Tb-MOFs are shown in Figure S6. Compared with the Tb-MOFs (inset Figure S6b), the RhB@Tb-MOFs showed a distinct new absorption peak at 550 nm (inset Figure S6c), which is the typical UV absorption peak of RhB (inset Figure S6a), further indicating that the RhB@Tb-MOFs were successfully synthesized [41].
The XPS spectra of the Tb-MOFs and RhB@Tb-MOFs show the Tb 3d "splitting peak" of the Tb-MOFs and RhB@Tb-MOFs in Figure S7a. Figure S7b,c show a shift from 1242.5 eV (Tb-MOFs) to 1243.4 eV (RhB@Tb-MOFs), suggesting a weak interaction between the N of RhB and Tb 3+ . Figure S7d shows the O 1s peak shift from 531.5 eV ( Figure S7e) to 532.5 eV ( Figure S7f) in the Tb-MOFs and RhB@Tb-MOFs. The higher binding energy of the RhB@Tb-MOFs was probably due to the successful coordination of the free carboxyl group of the RhB to the Tb 3+ of the Tb-MOFs, resulting in a decrease in the electron density of the O atom and an increase in the binding energy [42,43]. The above results confirmed the successful preparation of the RhB@Tb-MOFs [44].

Fluorescence Property of the RhB@Tb-MOFs Fluorescent Probe
The emission spectra of the Tb-MOFs are shown in Figure 2a. The CIE coordinate of (0.249, 0.566) is located in the green region, indicating that Tb-MOFs were green-emitting materials. As shown in Figure 2b, the RhB exhibited a characteristic red light emission peak centered at 605 nm, which is consistent with the reported RhB spectrum. The excitation spectra of the RhB@Tb-MOFs (Figure 2c) show ligand absorption and RhB absorption located at 226 nm and 256 nm, respectively, and these two peaks overlapped at 237 nm. The emission spectra of RhB@Tb-MOFs are shown in Figure 2d, which contains the characteristic emissions of Tb 3+ and RhB (blue-shift due to concentration and solvation effects [45]), indicating that the RhB@Tb-MOFs were successfully prepared. The color of the powder changed from white (Tb-MOFs) to purplish red (RhB@Tb-MOFs). The corresponding CIE coordinate of the RhB@Tb-MOFs was (0.413, 0.542), which lies between the green and red regions, indicating that the RhB@Tb-MOFs belonged to the green-red dual-emission composites.

RhB@Tb-MOFs Fluorescent Probe Continuously Detected Fe 3+ and AA
To investigate the ability of RhB@Tb-MOFs as fluorescent probes to detect metal ions, aqueous solutions of different metal ions (Fe 3+ , Ni 2+ , K + , Cu 2+ , Ca 2+ , Cr 3+ , Mg 2+ , Mn 2+ , Cd 2+ , Sr 2+ , Ba 2+ ) were added to suspensions of RhB@Tb-MOFs. The results are shown in Figure  3a. The RhB@Tb-MOFs showed different fluorescence depending on which of the 11 metal ions was tested. By comparison, Fe 3+ could remarkably quench the fluorescence of the RhB@Tb-MOFs. At the same time, the detection of the color change from orange-red to black could be directly observed with the naked eye. The histogram of the effect of the RhB@Tb-MOFs on the aqueous solution of different metal ions is intuitively presented in Figure 3b. The detection of Fe 3+ was the most prominent and superior to other metal ions,

RhB@Tb-MOFs Fluorescent Probe Continuously Detected Fe 3+ and AA
To investigate the ability of RhB@Tb-MOFs as fluorescent probes to detect metal ions, aqueous solutions of different metal ions (Fe 3+ , Ni 2+ , K + , Cu 2+ , Ca 2+ , Cr 3+ , Mg 2+ , Mn 2+ , Cd 2+ , Sr 2+ , Ba 2+ ) were added to suspensions of RhB@Tb-MOFs. The results are shown in Figure 3a. The RhB@Tb-MOFs showed different fluorescence depending on which of the 11 metal ions was tested. By comparison, Fe 3+ could remarkably quench the fluorescence of the RhB@Tb-MOFs. At the same time, the detection of the color change from orange-red to black could be directly observed with the naked eye. The histogram of the effect of the RhB@Tb-MOFs on the aqueous solution of different metal ions is intuitively presented in Figure 3b. The detection of Fe 3+ was the most prominent and superior to other metal ions, indicating that the RhB@Tb-MOFs could selectively detect Fe 3+ . The anti-interference detection of the RhB@Tb-MOFs was also carried out. When Fe 3+ was added to the RhB@Tb-MOFs suspensions treated with other metal ions, the emissions of the Tb 3+ and RhB were almost quenched entirely ( Figure S8a,b), similar to that with suspensions only treated with Fe 3+ , indicating that the RhB@Tb-MOFs were highly resistant to interference during the detection of Fe 3+ .
Molecules 2023, 28, x FOR PEER REVIEW 7 of 15 detection of the RhB@Tb-MOFs was also carried out. When Fe 3+ was added to the RhB@Tb-MOFs suspensions treated with other metal ions, the emissions of the Tb 3+ and RhB were almost quenched entirely ( Figure S8a,b), similar to that with suspensions only treated with Fe 3+ , indicating that the RhB@Tb-MOFs were highly resistant to interference during the detection of Fe 3+ . The sensing ability of Fe 3+ /RhB@Tb-MOFs for biomolecules was further investigated. Different solutions of biomolecules (alanine, serine, phenylalanine, tryptophan, proline, lysine, histidine, glycine, arginine, glucose, citric acid, tartaric acid and AA) were added to the Fe 3+ /RhB@Tb-MOFs and fluorescence tests were performed. As shown in Figure 3c, the RhB fluorescence of the Fe 3+ /RhB@Tb-MOFs was significantly restored after the addition of AA, and the results could also be directly distinguished by the naked eye. The histogram of the intensity (Figure 3d) shows the results of the Fe 3+ /RhB@Tb-MOFs for the detection of different biomolecules, with the best selective detection of AA. These results show that the RhB@Tb-MOFs could continuously detect Fe 3+ and AA and had infinite potential as ratiometric fluorescent probes for the continuous visualization detection of metal ions and biomolecules. The influence of potential disruptors on the Fe 3+ /RhB@Tb-MOFs was studied (Figure S8c), where AA significantly restored the fluorescence inten- The sensing ability of Fe 3+ /RhB@Tb-MOFs for biomolecules was further investigated. Different solutions of biomolecules (alanine, serine, phenylalanine, tryptophan, proline, lysine, histidine, glycine, arginine, glucose, citric acid, tartaric acid and AA) were added to the Fe 3+ /RhB@Tb-MOFs and fluorescence tests were performed. As shown in Figure 3c, the RhB fluorescence of the Fe 3+ /RhB@Tb-MOFs was significantly restored after the addition of AA, and the results could also be directly distinguished by the naked eye. The histogram of the intensity (Figure 3d) shows the results of the Fe 3+ /RhB@Tb-MOFs for the detection of different biomolecules, with the best selective detection of AA. These results show that the RhB@Tb-MOFs could continuously detect Fe 3+ and AA and had infinite potential as ratiometric fluorescent probes for the continuous visualization detection of metal ions and biomolecules. The influence of potential disruptors on the Fe 3+ /RhB@Tb-MOFs was studied ( Figure S8c), where AA significantly restored the fluorescence intensity at 570 nm in the Fe 3+ /RhB@Tb-MOFs without interference from others. The results indicate that the Fe 3+ /RhB@Tb-MOFs had high selectivity and strong anti-interference for the detection of AA.
The quantitative detection capability of the RhB@Tb-MOFs was further explored. The effect of different concentrations of Fe 3+ on the fluorescence intensity of the RhB@Tb-MOFs is displayed in Figure 4a

Detecting Mechanism
The structure of the RhB@Tb-MOFs, Fe 3+ /RhB@Tb-MOFs and AA+Fe 3+ /RhB@Tb-MOFs were tested using XRD and the results are displayed in Figure S9. The main diffraction peaks of the Fe 3+ /RhB@Tb-MOFs and AA+Fe 3+ /RhB@Tb-MOFs were not significantly different from the XRD of the RhB@Tb-MOFs. This result shows that the structures of RhB@Tb-MOFs remained relatively stable after detection, suggesting the mechanism of continuous detection of Fe 3+ and AA by RhB@Tb-MOFs did not result in a change in structure.
The fluorescence lifetime is another important verification to explore mechanism, the fluorescence lifetime tests were performed on the RhB@Tb-MOFs and Fe 3+ /RhB@Tb-MOFs.  (Figure 5c,d), respectively, both with reduced fluorescence lifetimes compared with the RhB@Tb-MOFs. The fluorescence lifetime was shortened due to the collision between the fluorophore and the quencher [46], indicating that the detection mechanism of the RhB@Tb-MOFs for Fe 3+ was a dynamic quenching process.

Detecting Mechanism
The structure of the RhB@Tb-MOFs, Fe 3+ /RhB@Tb-MOFs and AA+Fe 3+ /RhB@Tb-MOFs were tested using XRD and the results are displayed in Figure S9. The main diffraction peaks of the Fe 3+ /RhB@Tb-MOFs and AA+Fe 3+ /RhB@Tb-MOFs were not significantly different from the XRD of the RhB@Tb-MOFs. This result shows that the structures of RhB@Tb-MOFs remained relatively stable after detection, suggesting the mechanism of continuous detection of Fe 3+ and AA by RhB@Tb-MOFs did not result in a change in structure.
The fluorescence lifetime is another important verification to explore mechanism, the fluorescence lifetime tests were performed on the RhB@Tb-MOFs and Fe 3+ /RhB@Tb-MOFs. Figure 5a (Figure 5c,d), respectively, both with reduced fluorescence lifetimes compared with the RhB@Tb-MOFs. The fluorescence lifetime was shortened due to the collision between the fluorophore and the quencher [46], indicating that the detection mechanism of the RhB@Tb-MOFs for Fe 3+ was a dynamic quenching process. The internal filtration effect (IFE) is one of the mechanisms of fluorescence quenching and has been widely used in biochemical assays. As shown in Figure 6a, the excitation spectra of RhB@Tb-MOFs and the UV absorption spectra of Fe 3+ were tested, which overlapped in the short-wavelength range of 200~400 nm, and the energy transferred from the The internal filtration effect (IFE) is one of the mechanisms of fluorescence quenching and has been widely used in biochemical assays. As shown in Figure 6a, the excitation spectra of RhB@Tb-MOFs and the UV absorption spectra of Fe 3+ were tested, which overlapped in the short-wavelength range of 200~400 nm, and the energy transferred from the matrix to the luminescence center was absorbed by Fe 3+ , suggesting that the detection of Fe 3+ fluorescence quenching was likely due to the IFE [44,47]. In addition, the mechanism of the fluorescence recovery of RhB after the detection of AA by the Fe 3+ /RhB@Tb-MOFs was investigated by testing the UV-Vis absorption spectra of the AA solutions and a mixture of AA+Fe 3+ . As shown in Figure 6b, the UV-Vis absorption spectrum of the mixed AA+Fe 3+ solution was blue-shifted by 18 nm toward the shortwave wavelength compared with the AA solution, indicating a correlation reaction between the Fe 3+ and AA, which ultimately led to the recovery of fluorescence of the RhB [1].
cules 2023, 28, x FOR PEER REVIEW 10 of mixture of AA+Fe 3+ . As shown in Figure 6b, the UV-Vis absorption spectrum of the mixe AA+Fe 3+ solution was blue-shifted by 18 nm toward the shortwave wavelength compar with the AA solution, indicating a correlation reaction between the Fe 3+ and AA, whi ultimately led to the recovery of fluorescence of the RhB [1]. In order to further explain the internal mechanism of the RhB@Tb-MOFs fluoresce probe for the continuous detection of Fe 3+ and AA, XPS analysis was performed. Figu S10 shows the peak XPS spectra of the RhB@Tb-MOFs, Fe 3+ /RhB@Tb-MOFs an AA+Fe 3+ /RhB@Tb-MOFs (Tb 3d, O 1s and C 1s). To further analyze the changes in orbit valence, the "peak splitting" of Tb 3d and O 1s was tested. As shown in Figure 7a,b an 7c, the binding energy of Tb 3d at 1278.3 eV in the RhB@Tb-MOFs was shifted to 1276 eV in the Fe 3+ /RhB@Tb-MOFs, suggesting that Fe 3+ may have interacted with Tb 3+ in t RhB@Tb-MOFs and led to the fluorescence quenching. The binding energy of Tb 3d in t AA+Fe 3+ /RhB@Tb-MOFs was shifted to 1276.9 eV, with a partial fluorescence recovery RhB@Tb-MOFs as a result of the interaction of Fe 3+ with AA. As shown in Figure 7d,e the binding energy of the O 1s peak in the Fe 3+ /RhB@Tb-MOFs shifted about 0.7 eV com pared with that of the RhB@Tb-MOFs (from 532.5 eV to 531.8 eV), suggesting that F may have caused a change in the O 1s site in the RhB@Tb-MOFs. The binding energy of 1s of the AA+Fe 3+ /RhB@Tb-MOFs was also shifted 0.6 eV compared with the Fe 3+ /RhB@T MOFs (from 531.8 eV to 532.4 eV). This may have been due to the presence of sever oxygen-containing groups in the structure of AA, where it is likely that these O sites i teracted with Fe 3+ in some way, promoting partial fluorescence recovery of t Fe 3+ /RhB@Tb-MOFs. In order to further explain the internal mechanism of the RhB@Tb-MOFs fluorescent probe for the continuous detection of Fe 3+ and AA, XPS analysis was performed. Figure S10 shows the peak XPS spectra of the RhB@Tb-MOFs, Fe 3+ /RhB@Tb-MOFs and AA+Fe 3+ /RhB@Tb-MOFs (Tb 3d, O 1s and C 1s). To further analyze the changes in orbital valence, the "peak splitting" of Tb 3d and O 1s was tested. As shown in Figure 7a

Reagents and Instruments
All of the chemicals and solvents used in this work were commercially available in analytical grade and were used directly. Terbium nitrate hexahydrate (Tb ( The detailed characterization is given in the Supplementary Materials.

Synthesis of the Tb-MOFs and RhB@Tb-MOFs
The Tb-MOFs sample was synthesized using a fast and facile method at room temperature [48]. The RhB@Tb-MOFs sample was synthesized by using a simple ultrasonic immersion method. First, the 50 mg products of the Tb-MOFs, 5 mL of 1 mM ethanol solution of RhB was added into the 10 mL centrifuge tube, and then the mixture was shaken uniformly and equilibrated evenly for 30 min using an ultrasound treatment. Then, it was soaked for 48 h. Finally, the precipitates obtained were collected centrifugally and dried in an oven for 24 h.

Fluorescence Sensing for Detecting Fe 3+ and AA
In a typical operation, 3.0 mg of the RhB@Tb-MOFs sample powder was dispersed in a 10 mL centrifuge tube and ultrasound was performed for 10 min to obtain a standard stock solution of RhB@Tb-MOFs. Then, 5 mL of the RhB@Tb-MOFs original solution was
The detailed characterization is given in the Supplementary Materials.

Synthesis of the Tb-MOFs and RhB@Tb-MOFs
The Tb-MOFs sample was synthesized using a fast and facile method at room temperature [48]. The RhB@Tb-MOFs sample was synthesized by using a simple ultrasonic immersion method. First, the 50 mg products of the Tb-MOFs, 5 mL of 1 mM ethanol solution of RhB was added into the 10 mL centrifuge tube, and then the mixture was shaken uniformly and equilibrated evenly for 30 min using an ultrasound treatment. Then, it was soaked for 48 h. Finally, the precipitates obtained were collected centrifugally and dried in an oven for 24 h.

Fluorescence Sensing for Detecting Fe 3+ and AA
In a typical operation, 3.0 mg of the RhB@Tb-MOFs sample powder was dispersed in a 10 mL centrifuge tube and ultrasound was performed for 10 min to obtain a standard stock solution of RhB@Tb-MOFs. Then, 5 mL of the RhB@Tb-MOFs original solution was mixed with 1 mL of Fe 3+ solution with different concentrations (10 −2 -10 −5 M). The resulting solution was thoroughly mixed and its fluorescence emission spectra were measured under 237 nm excitation. In order to determine the concentration of ascorbic acid, 1 mL of Fe 3+ (10 −3 M) was introduced into the standard stock solution of the RhB@Tb-MOFs to form a sensitive and selective fluorescence sensing platform Fe 3+ /RhB@Tb-MOFs. Then, 1 mL of ascorbic acid with different concentrations (10-100 µM) was added, the mixture was shaken and incubated at room temperature for 30 min, and the fluorescence spectrum was measured.

Conclusions
In summary, a ratiometric fluorescent probe, namely, RhB@Tb-MOFs, with red-green dual emission was prepared and designed for excellent continuous sensing analysis of Fe 3+ and AA. The probe not only had the advantages of good selectivity, high sensitivity and high resistance to interference but also allowed for the detection results to be directly resolved by the naked eye. It could be used as an "off-on" fluorescent probe for the continuous identification of Fe 3+ and AA. In addition, the intrinsic mechanism of continuous recognition of Fe 3+ and AA using the RhB@Tb-MOFs was explored in detail. Based on this, the RhB@Tb-MOFs are expected to be a continuous fluorescent sensor for the detection of Fe 3+ in metal ions and AA in biological small molecules.